1. Field of the Invention
The present invention generally relates to wireless communications and, more particularly, to estimating noise and signal strength of RF signals.
2. Background of the Invention
FIG. 1 depicts a communication system 100 that implements wireless communications in accordance with the Institute of Electrical and Electronics Engineers (IEEE) 802.16 broadband wireless access standards (WiMax) for metropolitan area networks (MANs). WiMax specifies the use of orthogonal frequency division multiplexing (OFDM) as a modulation scheme to communicate data between a signal source, such as a basestation 110, and a subscriber station, such as a mobile station 120. OFDM is able to communicate a large amount of data over a limited bandwidth by allocating the data among multiple smaller sub-signals, and then simultaneously transmitting the sub-signals using different subcarriers.
Both carrier to interference and noise ratio (CINR) and receive signal strength indicator (RSSI) are important channel quality measurements for WiMax. In particular, IEEE 802.16e requires that each basestation 110 use CINR and RSSI measurements for modulation code scheme selection and tone/subchannel assignment. Although 802.16e does not mandate a particular method to be used for measuring CINR and RSSI, section 8.4.11 of the specification does recommend that RSSI and CINR be determined using the following equations:
                    RSSI        =                  C          ⁢                      1            N                    ⁢                                    ∑                              n                =                0                                            N                -                1                                      ⁢                                                                            y                                      k                    ,                    n                                                                              2                                                          (        1        )                                CINR        =                                            ∑                              n                =                0                                            N                -                1                                      ⁢                                                                            s                                      k                    ,                    n                                                                              2                                                          ∑                              n                =                0                                            N                -                1                                      ⁢                                                                                                x                                          k                      ,                      n                                                        -                                      s                                          k                      ,                      n                                                                                                  2                                                          (        2        )            where yk,n is the nth sample of a time-domain signal k that is output after analog to digital conversion, C is a constant that depends on the precision, input resistance and clip level of the analog to digital converter and the analog gain from an antenna with which the signal is received, xk,n is a received sample n within the signal k, sk,n represents an associated detected or pilot signal sample with channel state weighting, and N is the number of samples used in the estimate.
In real world systems, measurements of RSSI and CINR can be equivalently performed after the signals have been processed using the Fast Fourier Transform (FFT). Applying the recommended method to a FFT processed signal, RSSI and CINR are determined by the following equations:
                    RSSI        =                              C            ′                    ⁢                      1            N                    ⁢                                    ∑                              n                =                0                                            N                -                1                                      ⁢                                                  ⁢                                                        r                                  k                  ,                  n                                                                                                      (        3        )                                CINR        =                                            ∑                              n                =                0                                            N                -                1                                      ⁢                                                  ⁢                                                                                                p                                          k                      ,                      n                                                        ⁢                                      h                                          k                      ,                      n                                                                                                  2                                                          ∑                              n                =                0                                            N                -                1                                      ⁢                                                                                                r                                          k                      ,                      n                                                        -                                                            p                                              k                        ,                        n                                                              ⁢                                          h                                              k                        ,                        n                                                                                                                        2                                                          (        4        )            where C′ takes all path gain from analog to digital conversion and FFT into account (absent such gain, C′=C), rk,n is a received symbol on an nth subcarrier within the signal k after FFT, pk,n is a corresponding predetermined value of a pilot symbol, and hk,n is associated channel impulse response in the frequency domain.
In 802.16e, it is stated that the absolute accuracy of measurements shall be +/−4 dB for RSSI and +/−2 dB for CINR. Unfortunately, for real RF channels, the frequency domain coefficients of impulse response (hk,n) are unknown. Thus, the recommended equation for determining CINR is inadequate to insure that the CINR is measured on an FFT processed signal with the prescribed accuracy.